Floating platform



y 12, 9. cs. MONNEREAU ETAL 3,510,892

FLOATING PLATFORM Filed Nov. 29, 1967 I 2' Sheets-Sheet 1 May 12, 1970 e. MONNEREAU ETA!- 3,

FLOATING PLATFORM Filed Nov. 29, 1967 2 Sheets-Sheet 2 FIG. 5

fUnited States Patent Int. (:1. B 63b 21/52 US. Cl. 9-8 1 Claim ABSTRACT OF THE DISCLOSURE A floating platform comprising at least one semi-submersible Support unit, having a hollow vertical cylindrical shape and having a coaxial flat disc concentrically secured thereto and immersed to a specific depth below the sealevel for improving the stability to swells of the whole structure.

BACKGROUND OF THE INVENTION The present invention relates to vertical structure for semi-submersible platforms and concerns more particularly means for improving their stability to swells.

It is well known how advantageous it is to have stable floating platforms to be used as bases for observing moving objects, centers for oceanographic research or meteorological stations.

Moreover, activities connected with the petroleum industry carried out on the open sea have led to the constructions of sea platforms both for prospecting and for drilling.

Platforms used hitherto generally exhibited good stability. under ramming forces but were weighted in order to reduce the amount to which they would heel over under the action of the wind. They were then subjected by a swell to a considerable force causing them to oscillate to either side of the vertical.

The French Pat. No. 572,543, Armstrong for Improvements to Sea Stations" mentions vertical elements having a general cylindrical shape for the draught of a floating platform in which the floating elements are located at a specific place and between which are placed discs.

p The position of the discs in this patent is protected from the swells and enable the discs to have no other effect than to damp down the vertical movement of an element without being able to correct the oscillations about the vertical.

Furthermore, a floating device made of two distinct parts, one of which is located at the lower end of a vertical structure impedes the use of only one such vertical structure for supporting a simple platform.

SUMMARY OF THE INVENTION The present invention has for its object to eliminate such oscillating movements by arranging about the vertical axis of each cylindrical structure which participates in the platform supporting pressure, a horizontal immersed disc which has the same axis of symmetry. The resultant of the forces due to the swell and acting on the disc provide a force which opposes those exerted by the same swell at the same instant on the cylindrical walls of the structure. It is then suflicient in order to annul the action of the swell on the structure, to determine both the cross-sectional of the disc and the depth to which it must be immersed for this purpose.

A vertical structure for a semi-submersible platform which is swell-stable is characterised by a coaxial discoidal element of given cross-section which is immersed to a specific depth and fixed thereon.

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BRIEF DESCRIPTION OF THE DRAWINGS The aims and advantages of the present invention will be brought out from the following description associated with the drawings wherein:

FIG. 1 is a vertical perspective view showing a weighted hollow cylinder of the prior art type in equilibrium in the water;

FIG. 2 shows a similar view of the cylinder of FIG. 1 but in this case provided with a stabilising submerged disc;

FIG. 3 shows in horizontal plan view a group of three vertical structures with hexagonal discs assembled contiguously in tripod fashion;

FIG. 4 shows a horizontal plan view of a group of six structures with hexagonal discs assembled contiguously in hexapod formation; and

FIG. 5 shows a vertical schematic elevational view of a platform supported by one vertical structure only.

DETAILED DESCRIPTION OF THE INVENTION The hollow cylinder 1 in FIG. 1 which has a vertical axis zz', a cross-section o' and a closed lower end 2, is in equilibrium in a liquid whose means surface level is xx.

In the state of equilibrium, the submerged portion 3 has a length L and the emergent portion 4 is of a length l.

The cylinder 1 is subjected to a swell which is sinusoidal and, taking as the coordinate axes of the axes xx and yy' previously defined and intersecting at 0, the free surface of the liquid is expressed as follows:

z=a sin (kx-wt) a being the amplitude of the swell k=21r/ A being the wavelength of the swell w=21r/ T, T designating the period of the swell.

The action of the swell on the cylinder can be dissociated into two portions; one a vertical portion F and the other a horizontal portion F expressed respectively as:

When the submerged length L is sufiiciently great, the point of application I of the horizontal force F is situated on the axis 22' at a value Assuming that the submerged length L is of the same order of magnitude as the wavelength x, which is the case with the Cousteau-type laboratory tower or buoy, it is deduced that the amplitude of the vertical force is very slight and that the value of the point I is -'L/21r, that is to say about a sixth of the submerged length.

To obtain good static stability for the tower on either side of the vertical, the cylinder is weighted in order to lower the centre of gravity thereof sufliciently relative to the pressure centre P situated at the value -L/2.

Since the equations of movement are related to the centre of gravity G, the resultant movement relative thereto of the swell forces is therefore considerable. Taking into account the value of the length J, G and the horizontal swell force F an appreciable angular oscillation on either side of the vertical is created despite the considerable mass of the structure.

The action of the swell is felt very little towards the lower end of the cylinder and the instantaneous centre of rotation is a point on the axis of the cylinderwhich is situated further below the point G, so that the horizontal displacement of the platform situated at the other end is also accentuated, the whole process being as if the tower were being oscillated about its base.

The various points, which are easily verified in a swell tank, show clearly the inadequate stability of a tower of this type.

In FIG. 2 a flat disc 5 is added to the cylinder 1 which is horizontal and is submerged to a depth 6 below the mean surface level, of the value h. The disk 5 thus situated is subjected by a swell to a force opposing the force exerted by said swell on the cylinder 1.

Designating the speed of propagation of the swell by IL, the cross-section of the disc 5 subjected to the swell by E and the diameter of said disc by Q the disc is then subjected by the swell to a vertical resultant force having a moment relative to the point G which is proportional simultaneously to the three quantities; /2pp. (p being the specific gravity of the liquids), 2 q and that is to say proportional to the product of three quantities.

Since on the other hand ,u. =g/ k, g being the acceleration due to gravity and since k=21r/)\ in accordance with the approximation given hereinbefore, this gives for the said product a value proportional to zqmr Under the same conditions the moment, relative to the same point G, of the horizontal force applied at J and exerted by the swell on the cylinder 1, has the value aax] G.

A rapid examination of the pattern of variation of these quantities as a function of the parameter k shows that it is possible for a value k which is chosen to correspond to the most usual swell, Mediterranean or Atlantic, to obtain equality of the opposing moments.

For this, it is sufficient to adapt the position and dimensions of the submerged disc to the cylindrical structure being stabilised.

Experiments in a swell tank with a model to a scale of 91 has confirmed this point of view.

A disc twenty-five centimetres in diameter arranged fifteen centimetres below the mean surface level xx of the liquid permitted stabilisation of a cylinder of four centimetres in diameter weighted so as to have a submerged length of fifty centimetres. This is not a single solution, but one of the possible solutions.

On the other hand, a supplementary horizontal disc 7 fixed to the base of the tower makes it possible to improve the stability of the model with respect to the action of a swell.

The stabilising disc in fact protects the disc 7, the diameter of which is less than that of the former, from the action of the swell and the latter then only plays the part of a damping device serving on the one hand to increase the swell frequencies band in a known manner to permit stability and on the other hand to reduce the ramming effect.

For a simple observation platform, a single vertical cylindrical structure is sufficient, whereas to provide a larger surface such as for example in important oceonographic research, for fixed meteorlogical stations or for work connected with petroleum activities, a plurality of elements similar to FIG. 2 are used to support a more extensive platform.

The elements are connected to one another at two levels on the one hand at the upper ends thereof by the platform proper and on the other hand in the plane of the discs, which are all submerged to the same depth, by connecting these by any suitable means.

More particularly, in the case of using three elements, the discs can be given the form of regular hexagons and can be connected to one another in a contiguous manner.

A tripod association shown in FIG. 3 comprises three hexagonal discs 5' each mounted on a cylinder 1 having a vertical axis 22', each disc 5' being fixed in contiguous manner to the two others by the consecutive sides 8 and 9, respectively.

Similarly, FIG. 4 shows a hexapod association, that is to say comprising six discs with hexagonal peripheries each mounted on a cylinder 1 having a vertical axis 22. Each dis 5' is then connected in contiguous manner to the adjacent disc at a common side, 10 and 11 respectively. In this way the assembled group of six discs forms an inner surface 12 of regular hexagonal shape identical to that of the assembled hexagonal discs. The space 12 can then be provided, if appropriate, with a solid disc identical to a disc 5 and connected in contiguous manner at one side respectively to each of the six stabilising discs.

Although the present invention has been described in some detail, it should be understood that these are on y non limitative examples and that many changes in assembly details and in the combination and arrangement of the elements are possible without departing from the field of application of the stabilisation method proposed by the invention.

Thus, the vertical-axis cylindrical element instead of having a circular cross-section can be given any other cross-sectional shape allowing the vertical axis to be the axis of symmetry.

The discoidal element coaxial with the cylinder and providing the opposing force which is shown in plan in the drawings can equally well be conical or be given any other shape having as its axis of symmetry the axis of the cylinder, so as to be indifferent to the direction of the swell.

The discoidal elements illustrated are designed to fulfill two different functions; the upper element supplying the opposing forces for stabilisation and the lower element being used for providing a damping action. The discoidal elements can be modified so as to fulfill these two functions partly and simultaneously.

For this purpose, instead of being solid the discs may be apertured so as to facilitate the formation of many energy-dissipating vortices.

Furthermore, instead of being rigid, the discs can be given some elasticity by a suitable choice of the alloy of which they are constituted or by modifying their geometry, thus broadening the swell stability frequency band.

The tower shown in FIG. 5 comprises a single cylindrical structure 1', stabilised by means of a disc 7', and a two storied platform 13 and 14 topped by a telescopic platform 15 surrounded by radoms 16.

The cylinder 1, contrary to cylinder 1 does not extend under the disc 7 and twelve columns 18, especially placed around the edge of a regular dodecagon shaped disc 7', surround the chamber 1 which may be used as an engine-room and participate in supporting the platform.

The columns 19, also twelve in number, support the counterweight 20 intended to lower the centre of gravity of the tower. The lack of the hollow cylindrical part under the disc 7' combined with the hollow rigid shape of this latter allows the raising of the pressure center of the floating system to the immediate vicinity of the disc 7' so that the distance between this latter and the centre of buoyancy, corresponding to distance JG of FIG. 1, is sufiicient to give the tower a good stability to swells.

In order to summarily explain the part played by the disc, a dotted line 21 has been drawn symbolic of a swell which, during passage through the structure of the tower exerts on the one hand a horizontal force 22 on the cylinder 1' due to the difierence of pressure simultaneously applied to the right and to the left of the cylinder, on the other hand a vertical force 23 exerted on the disc 7' is due to the overpressure that is applied on the upper face of said disc with respect to the constant hydrostatic pressure prevailing on the lower face of the disc.

der 1.

For a tower having over-all dimensions of seventyfive metres high, the stabilising disc has a diameter of twenty-five metres, a distance of two and a half meters between its two horizontal faces and is immersed about twenty-five metres deep below the sea level.

For a bigger platform, several vertical structures identical to the preceding one would be added to the single vertical structure of the above mentioned example, and the stabilising discs would be far enough apart from each other to allow the compensating eflect proper to each one not to be disturbed.

What is claimed is:

1. A composite floating platform comprised of a plurality of elongated cylindrical floating supports each having a vertical axis of symmetry parallel to the others and equipped with angular deviation compensation means comprised of a discoidal element secured to each of said supports and disposed substantially perpendicular to and coaxial with its respective support, each of said discoidal elements being located at the same level, contiguous to and connected to the adjacent discoidal elements, the position of said level relative to the length of said supports and the total surface of said contiguous elements and the diameter of each of said supports being such that the vertical component of the hydrodynamic force exerted by the swell on the said total surface engenders a couple compensating for the disturbing torque resulting from the action of the horizontal component of the hydrodynamic force exerted by the swell on each of said floating supports.

References Cited UNITED STATES PATENTS 1,048,671 12/1912 Fidler 98 2,107,886 2/1938 Creed.

3,176,644 4/1965 Thomas et a1. 114126 3,191,202 6/1965 Handler 98 3,329,119 4/1967 Fritzsche 9--8 X TRYGVE M. BLIX, Primary Examiner 

